Also, as far as I understand the argument, it goes beyond "Canadian steel is a national security risk". A couple of years ago, Mexico was caught laundering $2B of Chinese aluminum to avoid US taxes.
http://fortune.com/2016/09/09/chinese-aluminum-giant-is-tied...
The theory, from the Trump crowd, is that Canada is also engaged in similar shady dealings with China. If true, that would put the US at risk.